1. Field of the Invention
The present invention relates to an optical fiber for preserving the plane of polarization. More specifically, the present invention relates to an optical fiber for preserving the plane of polarization which has excellent connection properties.
2. Background Art
These days, optical fibers for preserving the plane of polarization (hereinafter also referred to as polarization plane preserving optical fibers), which are capable of propagating light while maintaining two independent polarization states, are widely used in the field of optical communication and of photo sensors.
The polarization plane preserving optical fibers have a different refractive index in the transverse direction and the longitudinal direction so that a large difference in the propagation constant between the transverse and the longitudinal directions is generated and the mode coupling between the two polarization light becomes small.
A variety of types of polarization plane preserving optical fibers have been developed, and among them, the PANDA type optical fiber has a low degree of connection loss and is excellent for commercial production. The PANDA type optical fiber may be defined as an optical fiber that includes a stress imparting part that has a circular cross section and is made of a silica glass containing boron oxide (B2O3). An example of the cross section of the PANDA type optical fiber in its longitudinal direction is shown in FIG. 2.
In FIG. 2, the PANDA type optical fiber 10, which is an example of a polarization plane preserving optical fiber, includes a core 11, a clad 12, and stress imparting parts 13. Since the thermal expansion coefficient of each of the stress imparting parts 13, which is made of a silica glass doped with boron oxide, is a few times larger than that of a silica glass that does not include boron oxide, it shrinks when being subjected to a drawing process. At that time, one stress which acts to pull the core 11 in the direction connecting the two stress imparting parts 13 and another stress which acts to compress the core 11 in the vertical direction with respect to the above-mentioned stress are generated and maintained in the fiber. When these different stress are added, the refractive index of the optical fiber varies in the direction connecting the two stress imparting parts 13 and in the vertical direction thereto.
Due to the different refractive indexes in the two directions, the velocity of light propagating through the PANDA type optical fiber is varied. That is, the velocity of light which propagates through the PANDA type optical fiber 10 in the direction connecting the stress imparting parts 13 is slow, and this is called a slow axis or X-axis. On the other hand, the velocity of light which propagates through the PANDA type optical fiber 10 in the vertical direction with respect to the X-axis is fast, and this is called a fast axis or Y-axis.
However, since the viscosity of the stress imparting part 13 of the PANDA type optical fiber is smaller than that of the clad 12 which surrounds the stress imparting part 13, it is not easy to produce a core 11 having an ideal circular cross section, and usually, a non-circular cross section thereof tends to be produced during the fiber forming process at high temperature. If the cross section of the core 11 is non-circular, the degree of connection loss is increased when connecting these optical fibers.
On the other hand, in a single mode fiber, such as a polarization plane preserving optical fiber, light power is also propagated to portions of the clad 12 in the vicinity of the core 11. The magnitude of propagation towards the clad 12 is exponentially decreased, and as far as the single mode fiber is concerned, the mode field diameter (MFD), which expresses the degree of propagation of the light power in the optical fiber, has an important meaning as compared with a core size, which indicates a physical boundary in relation to the distribution of refractive index. Accordingly, it is thought that the connection characteristic of a polarization plane preserving optical fiber is greatly effected by the degree of non-circularity of the mode field diameter. Therefore, it is necessary to determine the degree of circularity of the mode field diameter which is sufficient to obtain a polarization plane preserving optical fiber having excellent connection properties.
The object of the present invention includes providing a polarization plane preserving optical fiber having a low connection loss by minimizing the degree of non-circularity of the mode field diameter.
The above object may be achieved by an optical fiber for preserving the plane of polarization, including: a core and a clad disposed so as to surround the core, wherein the mode field diameter non-circularity ratio of the optical fiber which is calculated according to the equation:       MFD    ⁢          xe2x80x83        ⁢    non    ⁢          -        ⁢    circularity    ⁢          xe2x80x83        ⁢    ratio    ⁢          xe2x80x83        ⁢          (      %      )        =                    Ma        -        Mb                                                        Ma              2                        +                          Mb              2                                2                      xc3x97    100  
where MFD indicates a mode field diameter, and Ma is the maximum value and Mb is the minimum value of the mode field diameter, respectively, in a wavelength region of light used), is about 3.2% or less to minimize a connection loss.
In another aspect of the invention, the core non-circularity ratio of the optical fiber, which is defined as a value obtained by dividing the difference between the diameter of a circumscribing circle and that of an inscribing circle of the core by the diameter of the core, is about 20% or less.
In yet another aspect of the invention, the optical fiber for preserving the plane of polarization is a PANDA type optical fiber.